Internet Engineering Task Force (IETF) X. Li
Request for Comments: 6219 C. Bao
Category: Informational M. Chen
ISSN: 2070-1721 H. Zhang
J. Wu
CERNET Center/Tsinghua
University
May 2011
The China Education and Research Network (CERNET) IVI TranslationDesign and Deployment for the IPv4/IPv6 Coexistence and Transition
Abstract
This document presents the China Education and Research Network
(CERNET)'s IVI translation design and deployment for the IPv4/IPv6
coexistence and transition.
The IVI is a prefix-specific and stateless address mapping mechanism
for "an IPv6 network to the IPv4 Internet" and "the IPv4 Internet to
an IPv6 network" scenarios. In the IVI design, subsets of the ISP's
IPv4 addresses are embedded in the ISP's IPv6 addresses, and the
hosts using these IPv6 addresses can therefore communicate with the
global IPv6 Internet directly and can communicate with the global
IPv4 Internet via stateless translators. The communications can
either be IPv6 initiated or IPv4 initiated. The IVI mechanism
supports the end-to-end address transparency and incremental
deployment. The IVI is an early design deployed in the CERNET as a
reference for the IETF standard documents on IPv4/IPv6 stateless
translation.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6219.
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RFC 6219 CERNET IVI Translation Design May 20111. Introduction
This document presents the CERNET IVI translation design and
deployment for the IPv4/IPv6 coexistence and transition. In Roman
numerals, the "IV" stands for 4, and "VI" stands for 6, so "IVI"
stands for the IPv4/IPv6 translation.
The experiences with IPv6 deployment in the past 10 years indicate
that the ability to communicate between IPv4 and IPv6 address
families would be beneficial. However, the current transition
methods do not fully support this requirement [RFC4213]. For
example, dual-stack hosts can communicate with both the IPv4 and IPv6
hosts, but single-stack hosts can only communicate with hosts in the
same address family. While the dual-stack approach continues to work
in many cases even in the face of IPv4 address depletion [COUNT],
there are situations where it would be desirable to communicate with
a device in another address family. Tunneling-based architectures
can link the IPv6 islands across IPv4 networks, but they cannot
provide communication between the two different address families
[RFC3056] [RFC5214] [RFC4380]. Translation can relay communications
for hosts located in IPv4 and IPv6 networks, but the current
implementation of this kind of architecture is not scalable, and it
cannot maintain end-to-end address transparency [RFC2766] [RFC3142]
[RFC4966] [RFC2775].
1.1. Analysis of IPv4-IPv6 Translation Mechanisms
Since IPv4 and IPv6 are different protocols with different addressing
structures, a translation mechanism is necessary for communication
between endpoints using different address families. There are
several ways to implement the translation. One is the Stateless IP/
ICMP Translation Algorithm (SIIT) [RFC2765], which provides a
mechanism for translation between IPv4 and IPv6 packet headers
(including ICMP headers) without requiring any per-connection state.
However, SIIT does not specify the address assignment and routing
scheme [RFC2766]. For example, SIIT uses IPv4-mapped IPv6 addresses
[::ffff:ipv4-addr/96] and IPv4-compatible IPv6 addresses
[::ipv4-address/96] for the address mapping, but these addresses
violate the aggregation principle of IPv6 routing [RFC4291]. The
other translation mechanism is Network Address Translation - Protocol
Translation (NAT-PT), which has serious technical and operational
difficulties; the IETF has reclassified it from Proposed Standard to
Historic status [RFC4966].
In order to solve the technical difficulties in NAT-PT, the issues
and the possible workarounds are:
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RFC 6219 CERNET IVI Translation Design May 2011
1. NAT-PT disrupts all protocols that embed IP addresses (and/or
ports) in packet payloads. There is little that can be done
about this, other than using Application Layer Gateways (ALGs) or
preferring protocols that transport DNS names instead of
addresses.
2. Loss of end-to-end address transparency may occur. End-to-end
address transparency implies a global address space, the ability
to pass packets unaltered throughout the network, and the ability
to use source and destination addresses as unique labels
[RFC2775]. A reversible, algorithmic mapping can restore some of
this transparency. However, it is still not possible to ensure
that all nodes in the existing Internet support such reversible
mappings.
3. The states maintained in the translator cause scalability,
multihoming, and load-sharing problems. Hence, a stateless
translation scheme is preferred.
4. Loss of information due to incompatible semantics between IPv4
and IPv6 versions of headers and protocols may occur. A partial
remedy to this is the proper attention to the details of the
protocol translation, for example, the error-codes mapping
between ICMP and ICMPv6. However, some semantic differences
remain.
5. The DNS is tightly coupled with the translator and lack of
address mapping persistence discussed in Section 3.3 of
[RFC4966]. Hence, the DNS should be decoupled from the
translator.
6. Support for referrals is difficult in NAT-PT, given that
translated addresses may leak outside the network where these
addresses have a meaning. Stateless translation, algorithmic
address mappings, and the decoupling of DNS from the translation
process can help the handling of referrals. Nevertheless, it is
still possible that an address-based referral is passed to
someone who cannot employ it. For instance, an IPv6-only node
may pass a referral based on an IPv6 address to a node that only
understands IPv4.
1.2. CERNET Translation Requirements
The China Education and Research Network has two backbones using
different address families. The CERNET is IPv4-only [CERNET] and
CERNET2 is IPv6-only [CNGI-CERNET2], which fit in "an IPv6 network to
the IPv4 Internet" and "the IPv4 Internet to an IPv6 network"
scenarios in the IETF BEHAVE working group definition [BEHAVE]
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RFC 6219 CERNET IVI Translation Design May 2011
[RFC6144]. In order to make CERNET2 communicate with the IPv4
Internet, we designed the IVI mechanism and installed IVI translators
between the CERNET and CERNET2.
The requirements of the IVI mechanism are:
1. It should support both IPv6-initiated and IPv4-initiated
communications for the IPv6 clients/servers in "an IPv6 network".
2. It should follow current IPv4 and IPv6 routing practice without
increasing the global routing table size in both address
families.
3. It should be able to be deployed incrementally.
4. It should be able to use IPv4 addresses effectively due to the
IPv4 address depletion problem.
5. It should be stateless to achieve scalability.
6. The DNS function should be decoupled from the translator.
The specific IVI design presented in this document can satisfy the
above requirements, with the following notes:
1. It restricts the IPv6 hosts to use a subset of the addresses
inside the ISP's IPv6 block. Therefore, IPv6 autoconfiguration
cannot be used for these IPv6 hosts. Manual configuration or
autoconfiguration via stateful DHCPv6 is required.
2. It defines a one-to-one mapping between IPv4 addresses and IPv6
addresses; hence, the IPv4 addresses cannot be used efficiently.
However, the IVI6 addresses can be used both for IPv6 clients and
IPv6 servers. Due to this limitation, we suggest using IVI6
addresses for servers.
3. An ALG is still required for any applications that embed
address(es) in the payload.
4. Some issues with end-to-end transparency, address referrals, and
incompatible semantics between protocol versions still remain, as
discussed above.
The IVI is an early design deployed in the CERNET for the stateless
translation. The IETF standard IPv4-IPv6 stateless and stateful
translation mechanisms are defined in [RFC6144], [RFC6052],
[RFC6145], [RFC6146], and [RFC6147].
Li, et al. Informational [Page 5]

RFC 6219 CERNET IVI Translation Design May 20112. Terms and Abbreviations
The following terms and abbreviations are used in this document:
ISP(i): A specific Internet service provider "i".
IVIG4: The global IPv4 address space.
IPS4(i): A subset of IVIG4 allocated to ISP(i).
IVI4(i): A subset of IPS4(i); the addresses in this set will be
mapped to IPv6 via the IVI mapping mechanism and used by IPv6
hosts of ISP(i).
IPG6: The global IPv6 address space.
IPS6(i): A subset of IPG6 allocated to ISP(i).
IVIG6(i): A subset of IPS6(i), and an image of IVIG4 in the IPv6
address family via the IVI mapping mechanism. It is defined as
the IPv4-converted address in [RFC6144].
IVI6(i): A subset of IVIG6(i) and an image of IVI4(i) in the IPv6
address family via the IVI mapping mechanism. It is defined as
the IPv4-translatable address in [RFC6144].
IVI translator: The mapping and translation gateway between IPv4 and
IPv6 based on the IVI mechanism.
IVI DNS: Providing the IVI Domain Name System (DNS).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL", when
they appear in this document, are to be interpreted as described in
[RFC2119].
3. The IVI Translation Algorithm
The IVI is a prefix-specific and stateless address mapping scheme
that can be carried out by individual ISPs. In the IVI design,
subsets of the ISP's IPv4 addresses are embedded in the ISP's IPv6
addresses, and the hosts using these IPv6 addresses can therefore
communicate with the global IPv6 Internet directly and can
communicate with the global IPv4 Internet via stateless translators.
The communications can either be IPv6 initiated or IPv4 initiated.
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RFC 6219 CERNET IVI Translation Design May 2011
The IVI mapping and translation mechanism is implemented in an IVI
translator that connects between "an IPv6 network" and the IPv4
Internet via the ISP's IPv4 network, as shown in the following
figure.
------ ----- ------
/ The \ ----- / An \ / The \
| IPv4 |-----|Xlate|------| IPv6 |-----| IPv6 |
\Internet/ ----- \Network/ \Internet/
------ ----- ------
<===>
Figure 1: The Scenarios: "An IPv6 Network to the IPv4 Internet" and
"the IPv4 Internet to an IPv6 Network"
In order to perform the translation function between IPv4 and IPv6
addresses, the translator needs to represent the IPv4 addresses in
IPv6 and the IPv6 addresses in IPv4.
To represent the IPv4 addresses in IPv6, a unique, prefix-specific,
and stateless mapping scheme is defined between IPv4 addresses and
subsets of IPv6 addresses, so each provider-independent IPv6 address
block (usually a /32) will have a small portion of IPv6 addresses
(for example, /40 defined by PREFIX), which is the image of the
totality of the global IPv4 addresses, as shown in the following
figure. The SUFFIX is all zeros.
+-+-+-+-+-+-+
| IVIG4 |
+-+-+-+-+-+-+
||
\ /
\/
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PREFIX | IPv4 addr | SUFFIX |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 2: Representing the IPv4 Addresses in IPv6
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RFC 6219 CERNET IVI Translation Design May 2011
To represent the IPv6 addresses in IPv4, each provider can borrow a
portion of its IPv4 addresses and map them into IPv6 based on the
above mapping rule. These special IPv6 addresses will be physically
used by IPv6 hosts. The original IPv4 form of the borrowed addresses
is the image of these special IPv6 addresses, and it can be accessed
by the IPv4 Internet, as shown in the following figure. The SUFFIX
can either be all zeros, or some other value for future extensions.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PREFIX | |IVI4| | SUFFIX |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
||
\ /
\/
-+-+-+
|IVI4|
-+-+-+
Figure 3: Representing the IPv6 Addresses in IPv4
3.1. Address Format
The IVI address format is defined based on an individual ISP's IPv6
prefix, as shown in the following figure
| 0 |32 |40 |72 127|
------------------------------------------------------------------
| |ff | | |
------------------------------------------------------------------
|<- PREFIX ->|<- IPv4 address ->| <- SUFFIX -> |
Figure 4: IVI Address Mapping
where bit 0 to bit 31 are the prefix of ISP(i)'s /32 (e.g., using
document IPv6 address IPS6=2001:db8::/32) in the CERNET
implementation, bit 32 to bit 39 are all ones as the identifier of
the IVI addresses, and bit 40 to bit 71 are embedded global IPv4
space (IVIG4), presented in hexadecimal format (e.g.,
2001:db8:ff00::/40). Note that based on the IVI mapping mechanism,
an IPv4 /24 is mapped to an IPv6 /64, and an IPv4 /32 is mapped to an
IPv6 /72.
The IETF standard for the address format is defined in [RFC6052].
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RFC 6219 CERNET IVI Translation Design May 20113.2. Routing and Forwarding
Based on the IVI address mapping rule, routing is straightforward, as
shown in the following figure
/-----\ /-----\
( ISP's ) -- 192.0.2.2 ----------- 2001:db8::2 -- ( ISP's )
( IPv4 )--|R1|-------------| IVI Xlate |------------|R2|---( IPv6 )
(network) -- 192.0.2.1 ----------- 2001:db8::1 -- (network)
\-----/ \-----/
| |
| |
The IPv4 Internet The IPv6 Internet
Figure 5: IVI Routing
where
1. IVI Xlate is a special dual-stack router, with two interfaces,
one to the IPv4 network and the other to the IPv6 network (it is
also possible to have a single interface configured with both
IPv4 and IPv6 addresses). IVI Xlate can support dynamic routing
protocols in IPv4 and IPv6 address families. In the above
configuration, the static routing configuration can be used.
2. Router R1 has an IPv4 route for IVI4(i)/k (k is the prefix length
of IVI4(i)) with the next hop equal to 192.0.2.1, and this route
is distributed to the Internet with proper aggregation.
3. Router R2 has an IPv6 route for IVIG6(i)/40 with the next hop
equal to 2001:db8::1, and this route is distributed to the IPv6
Internet with proper aggregation.
4. The IVI translator has an IPv6 route for IVI6(i)/(40+k) with the
next hop equal to 2001:db8::2. The IVI translator also has an
IPv4 default route 0.0.0.0/0 with the next hop equal to
192.0.2.2.
Note that the routes described above can be learned/inserted by
dynamic routing protocols (IGP or BGP) in the IVI translator peering
with R1 and R2.
Since both IVI4(i) and IVI6(i) are aggregated to IPS4(i) and IPS6(i)
in ISP(i)'s border routers, respectively, they will not affect the
global IPv4 and IPv6 routing tables [RFC4632].
Since the IVI translation is stateless, it can support multihoming
when the same prefix is used for multiple translators.
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RFC 6219 CERNET IVI Translation Design May 2011
The IETF standard for IP/ICMP translation is defined in [RFC6145],
which contains updated technical specifications.
3.4. Transport-Layer Header Translation
Since the TCP and UDP headers [RFC0793] [RFC0768] consist of
checksums that include the IP header, the recalculation and updating
of the transport-layer headers MUST be performed. Note that SIIT
does not recalculate the transport-layer checksum, since checksum-
neutral IPv6 addresses are used in SIIT [RFC2765].
The IETF standard for transport-layer header translation is defined
in [RFC6145], which contains updated technical specifications.
3.5. Fragmentation and MTU Handling
When the packet is translated by the IVI translator, due to the
different sizes of the IPv4 and IPv6 headers, the IVI6 packets will
be at least 20 bytes larger than the IVI4 packets, which may exceed
the MTU of the next link in the IPv6 network. Therefore, the MTU
handling and translation between IPv6 fragmentation headers and the
fragmentation field in the IPv4 headers are necessary; this is
performed in the IVI translator according to SIIT [RFC2765].
The IETF standard for fragmentation and MTU handling is defined in
[RFC6145], which contains updated technical specifications.
3.6. ICMP Handling
For ICMP message translation between IPv4 and IPv6, IVI follows the
ICMP/ICMPv6 message correspondence as defined in SIIT [RFC2765].
Note that the ICMP message may be generated by an intermediate router
whose IPv6 address does not belong to IVIG6(i). Since ICMP
translation is important to the path MTU discovery and
troubleshooting, the IPv4 representation of the non-IVIG6 addresses
in the ICMP packets is required. In the current IVI prototype, a
small IPv4 address block is used to identify the non-IVIG6 addresses.
This prevents translated ICMP messages from being discarded due to
unknown or private IP sources.
The IETF standard for IP/ICMP translation is defined in [RFC6145],
which contains updated technical specifications.
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RFC 6219 CERNET IVI Translation Design May 20113.7. Application Layer Gateway
Due to the features of 1-to-1 address mapping and stateless
operation, IVI can support most of the existing applications, such as
HTTP, Secure SHell (SSH), and Telnet. However, some applications are
designed such that IP addresses are used to identify application-
layer entities (e.g., FTP). In these cases, an Application Layer
Gateway (ALG) is unavoidable, and it can be integrated into the IVI
translator.
The discussion of the use of ALGs is in [RFC6144].
4. The IVI DNS Configuration
The DNS [RFC1035] service is important for the IVI mechanism.
4.1. DNS Configuration for the IVI6(i) Addresses
For providing authoritative DNS service for IVI4(i) and IVI6(i), each
host name will have both an A record and a AAAA record pointing to
IVI4(i) and IVI6(i), respectively. Note that the same name always
points to a unique host, which is an IVI6(i) host, and it has IVI4(i)
representation via the IVI translator.
4.2. DNS Service for the IVIG6(i) Addresses
For resolving the IPv6 form of the global IPv4 space (IVIG6(i)), each
ISP must provide customized IVI DNS service for the IVI6(i) hosts.
The IVI DNS server MUST be deployed in a dual-stack environment.
When the IVI6(i) host queries a AAAA record for an IPv4-only domain
name, the IVI DNS will query the AAAA record first. If the AAAA
record does not exist, the IVI DNS will query the A record and map it
to IVIG6(i), and return a AAAA record to the IVI6(i) host. The
technical specifications for this process are defined in [RFC6147].
5. The Advanced IVI Translation Functions5.1. IVI Multicast
The IVI mechanism can support IPv4/IPv6 communication of Protocol
Independent Multicast - Source-Specific Multicast (PIM-SSM) [RFC5771]
[RFC3569] [RFC4607].
There will be 2^24 group addresses for IPv4 SSM. The corresponding
IPv6 SSM group addresses can be defined as shown in the following
figure.
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RFC 6219 CERNET IVI Translation Design May 2011
-------------------------------------------------------
IPv4 Group Address IPv6 Group Address
-------------------------------------------------------
232.0.0.0/8 ff3e:0:0:0:0:0:f000:0000/96
232.255.255.255/8 ff3e:0:0:0:0:0:f0ff:ffff/96
-------------------------------------------------------
Figure 8: IVI Multicast Group Address Mapping
The source address in IPv6 MUST be IVI6(i) in order to perform
Reverse Path Forwarding (RPF) as required by PIM - Sparse Mode
(PIM-SM).
The interoperation of PIM-SM for IPv4 and IPv6 address families can
either be implemented via an Application Layer Gateway or via static
joins based on IGMPv3 and Multicast Listener Discovery Version 2
(MLDv2) in IPv4 and IPv6, respectively.
6. IVI Host Operation6.1. IVI Address Assignment
The IVI6 address has a special format (for example, IVI4=192.0.2.1/32
and IVI6=2001:db8:ffc0:2:100::/72); therefore, stateless IPv6 address
autoconfiguration cannot be used. However, the IVI6 can be assigned
to the IPv6 end system via manual configuration or stateful
autoconfiguration via DHCPv6.
o For the manual configuration, the host needs to configure the IVI6
address and the corresponding prefix length, as well as the
default gateway address and the DNS resolver address.
o For the DHCPv6 configuration, the DHCPv6 will assign the IVI6
address and the DNS resolver address to the host. The router in
the subnet should enable router advertisement (RA), since the
default gateway is learned from the router.
6.2. IPv6 Source Address Selection
Since each IPv6 host may have multiple addresses, it is important for
the host to use an IVI6(i) address to reach the global IPv4 networks.
The short-term workaround is to use IVI6(i) as the default source
IPv6 address of the host, defined as the policy table in [RFC3484].
The long-term solution requires that the application should be able
to select the source addresses for different services.
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RFC 6219 CERNET IVI Translation Design May 20117. The IVI Implementation7.1. Linux Implementation
An implementation of IVI exists for the Linux operating system. The
source code can be downloaded from [LINUX]. An example of how to
configure an IVI deployment is shown in Appendix A.
The IVI DNS source code for the IVIG6(i) addresses presented in this
document can be downloaded from [DNS].
7.2. Testing Environment
The IVI translator based on the Linux implementation has been
deployed between [CERNET] (IPv4-only) and [CNGI-CERNET2] (IPv6-only)
since March 2006. The pure-IPv6 web servers using IVI6 addresses
[2001:250:ffca:2672:100::] behind the IVI translator can be accessed
by the IPv4 hosts [TEST4], and also by the global IPv6 hosts [TEST6].
The pure-IPv6 clients using IVI6 addresses behind the IVI translator
can access IPv4 servers on the IPv4 Internet.
Two traceroute results are presented in Appendix B to show the
address mapping of the IVI mechanism.
IVI6 manual configuration and DHCPv6 configuration of the IPv6 end
system have also been tested with success.
8. Security Considerations
This document presents the prefix-specific and stateless address
mapping mechanism (IVI) for the IPv4/IPv6 coexistence and transition.
The IPv4 security and IPv6 security issues should be addressed by
related documents of each address family and are not included in this
document.
However, there are several issues that need special considerations,
specifically (a) IPsec and its NAT traversal, (b) DNS Security
Extensions (DNSSEC), and (c) firewall filter rules.
o IPsec and its NAT traversal: Since the IVI scheme maintains end-
to-end address transparency, IPsec could work with or without NAT
traversal techniques.
o DNSSEC: DNSSEC verification will be terminated at the IVI DNS for
the "A record to AAAA record" translation. It would be fine to
have a translation in a local IVI DNS server that also verifies
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RFC 6219 CERNET IVI Translation Design May 2011
DNSSEC, or in the host, if the host both translates the DNS entry
and again verifies DNSSEC validity. The DNSSEC discussion is in
[RFC6147].
o Firewall filter rules: Since the IVI scheme maintains the end-to-
end address transparency and there is a unique mapping between
IPv4 and IPv6 addresses, the firewall filter rule can therefore be
implemented for one address family, or mapped to another address
family and implemented in that address family. However, the
current IPv6 routers may only support the access-list or uRPF
(unicast Reverse Path Forwarding) for the prefix length shorter
than /64; there may a practical constraint for the construction of
such rules.
Except for the issues discussed above, we have not found special
security problems introduced by the IVI translation in our
experiments.
9. Contributors
The authors would like to acknowledge the following contributors in
the different phases of the IVI development: Ang Li, Yuncheng Zhu,
Junxiu Lu, Yu Zhai, Wentao Shang, Weifeng Jiang, and Bizheng Fu.
The authors would like to acknowledge the following contributors, who
provided helpful inputs concerning the IVI concept: Bill Manning,
David Ward, Elwyn Davies, Lixia Zhang, Jun Murai, Fred Baker, Jari
Arkko, Ralph Droms, Tony Hain, and Kevin Yin.
10. Acknowledgments
The authors thank the following for funding support: the CERNET,
CNGI-CERNET2, CNGI Research and Development, and the China "863" and
China "973" projects.
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